<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <style>
    div{
        width: 300px;
        background-color: rgba(0,0,0,0.3);
        padding: 10px;/*四个方向的内边距 实现缩进效果*/
        position: relative;
    }
    input{
        width: 260px;/*300-40内边距*/
        border: 0;
        padding: 10px 20px;
    }
    img{
        position: absolute;
        right: 20px;
        top: 14px;
    }
    p{
        font-size: 12px;
        color: red;
        margin: 0;
    }

  </style>
</head>
<body>
<div>

<input type="text" placeholder="请输入用户名">
<img src="http://doc.canglaoshi.org/tstore_v1/images/login/yhm.png" alt="">
<p>用户名不能为空</p>
</div>
</body>
</html>